1. Field of the Invention
This invention relates to a process for degasifying sulfur. More particularly, this invention relates to a multistage process for continuously removing H.sub.2 S from H.sub.2 S-rich liquid sulfur.
2. Description of the Prior Art
In several petroleum refining processes, sulfur is removed from various hydrocarbon materials by reaction with hydrogen to form H.sub.2 S. H.sub.2 S is also present in sour natural gas. In either case, the H.sub.2 S is usually removed therefrom by absorption in a solution of an alkanolamine and subsequently released in concentrated form when the solution is heated. The H.sub.2 S may then be converted to elemental sulfur by the well known Claus process (see Kirk Othmer, The Encyclopedia of Chemical Technology, 2nd Ed., Vol. 4, p. 376-377, 1964, and Vol. 19, p. 353-354, 1969). Briefly, in this process, part (about 1/3) of the concentrated H.sub.2 S is combusted with air to form SO.sub.2 which then reacts with the remaining H.sub.2 S at about 300.degree. C. in the presence of a catalyst to form gaseous sulfur. Typically, three stages of combustion, i.e. H.sub.2 S conversion, are employed, each stage operating at a progressively lower temperature and containing progressively reduced amounts of H.sub.2 S, such that the conversion to sulfur approaches equilibrium. Using three stages, about 95% of the H.sub.2 S in the gas feed to the Claus plant is converted to sulfur. Following each stage, the sulfur is cooled and the three liquid sulfur streams thus formed are combined into an H.sub.2 S-rich liquid sulfur stream which is then passed to a sulfur pit which serves to store the sulfur until further disposition. Typically, the H.sub.2 S-rich liquid sulfur will contain from about 70 to about 300 wppm H.sub.2 S, including hydrogen polysulfides having the formula H.sub.2 S.sub.x where x ranges from 1-6. For the sake of convenience, the H.sub.2 S equivalent to the H.sub.2 S present in the H.sub.2 S lean liquid sulfur as H.sub.2 S and as H.sub.2 S.sub.x will be termed hereinafter "equivalent H.sub.2 S."
Aside from being a noxious gas in relatively dilute concentrations, H.sub.2 S becomes progressively dangerous as its concentration in a gas exceeds about 70 wppm, the fatal concentration for man being about 700 wppm. When the H.sub.2 S content of a gas increases to at least 3.6 volume % (which corresponds to an equilibrium value of 15 wppm H.sub.2 S in liquid sulfur) in the presence of an oxygen-containing gas such as air, the inflammability threshold of the gas will be exceeded. In addition, should liquid sulfur be confined in a vessel having iron as a material of construction, the iron sulfide formed on the inner walls thereof may be pyrophoric and, in the presence of an oxygen-containing gas, may result in spontaneous ignition of H.sub.2 S without any flame or spark being present. Thus, it is desirable to maintain the concentration of equivalent H.sub.2 S in liquid sulfur below 15 wppm.
One method for removing H.sub.2 S from H.sub.2 S-rich liquid sulfur has been suggested in U.S. Pat. No. 3,364,655. According to the method disclosed therein, a batch of the sulfur is passed into a separate degasification vessel and recirculated through spray nozzles such that the sulfur spray thus formed is contacted with a gas in the vapor space above the liquid sulfur. However, this process requires expenditures for a separate degasification vessel and suffers from the economic and operational disadvantages normally associated with most batch processes, e.g. not supplying a product continuously, larger storage requirements during processing, more operator attention and/or instrumentation, increased maintenance and the like.
Another method for removing H.sub.2 S from H.sub.2 S-rich liquid sulfur is disclosed in U.S. Pat. No. 3,920,424. According to the process described therein, a batch of H.sub.2 S-rich liquid sulfur is recirculated continuously from a storage tank through a vertical gas scrubbing apparatus until the desired H.sub.2 S content of the liquid sulfur has been obtained. The scrubbing apparatus contains baffle plates which serve to increase the surface area of the downwardly flowing liquid sulfur, thereby facilitating the escape of the H.sub.2 S from the sulfur. Again, however, this process suffers from the disadvantages associated with most batch processes.
Yet another method for degasifying liquid sulfur is disclosed in British Pat. No. 1,402,274 wherein a single stage continuous process is employed. However, it is believed that the present multistage process is not suggested by the prior art.